This invention concerns radiation therapy, especially brachytherapy, for treating tissues which may have diffuse proliferative disease.
In brachytherapy, the radiation source is generally placed within a surgically created or naturally occurring cavity in the body. In particular, this invention relates to delivery of radiation therapy to glandular and adipose tissue as might be found in the human breast, or to other similar tissue which can be easily manipulated, often following surgical treatment of cancer.
Radiation therapy following tumor resection or partial resection is generally administered over a period of time in partial doses, or fractions, the sum of which comprises a total prescribed dose. This fractional application takes advantage of cell recovery differences between normal and cancerous tissue whereby normal tissue tends to recover between fractions, while cancerous tissue tends not to recover.
In brachytherapy, a prescribed dose is selected by the therapist to be administered to a volume of tissue (the target tissue) lying outside the treatment cavity into which the radiation source will be placed. Generally the prescribed dose will include a minimum dose to be delivered at a preferred depth outside the treatment cavity (the prescription depth). Since, in accordance with the laws of physics, radiation intensity falls off sharply with increasing distance from the radiation source, it is desirable to create and maintain a space between the source of radiation and the first tissue surface to be treated (generally the cavity wall since the source is placed within the cavity) in order to moderate the absorbed dose at the cavity surface. This is often done by placing a balloon or other applicator in the cavity with the radiation source inside the applicator.
Although not always the case, generally the absorbed dose at the prescription depth outside the cavity is to be uniform. In this isotropic case, it is therefore important that the incident radiation on the interior surface of the cavity be the same at all points being treated. To accomplish this objective, it may be necessary to sequentially position a single radiation source through a series of positions (or utilize multiple sources strategically placed) which, in the aggregate, produce a uniform absorbed dose incident on the cavity surface being treated. When this is achieved, the absorbed dose reaching into tissue will be the same at all points being treated, and the minimum prescribed dose can be delivered at the prescription depth as nearly as the treatment plan will allow. Furthermore, by selecting the radiation source intensity (radioisotope emissions or x-ray tube output) and controlling treatment time and the distance from the source(s) to the cavity interior surface, the incident radiation can be sufficiently moderated to avoid substantial damage to normal tissue.
In many instances, the treatment cavity may be near the skin such that the treatment plan, if isotropic, may include points at the prescription depth which lie outside the skin surface. If prescription depth at those points lies far enough outside the skin, too high a dose could be received at the skin, and the therapist may be forced to locally shield emitted radiation from within the treatment cavity (see copending application Ser. No. 11/471,277 incorporated by reference herein in its entirety), or to resort to anisotropic radiation sources. Such capabilities may not be readily available or practical. In such cases, and there are many, the patient may therefore be denied the advantages of brachytherapy.
One currently accepted standard in radiation therapy is that, for a one centimeter prescription depth of tissue, and for the applicator diameter range of intent, assuming the tissue at the prescription depth receives the desired dose (1×), the tissue nearest the source should not receive more than 2.5× to 3× the prescription dose. Standards also usually require that the skin not receive a dose of more than 1.5× the prescription dose. With a one centimeter prescription depth, this usually requires the skin be at least 6-8 mm out from the surface of a balloon applicator engaged against the tissue in a cavity. A distance of less than about 6-8 mm may result in doses higher than 1.5× the prescription dose which are known to often result in undesirable cosmesis. This problem commonly arises in breast brachytherapy and is a counter-indication for brachytherapy. In order to make brachytherapy available to more patients having resection cavities in close proximity to skin surfaces or to other radiation sensitive structures, the apparatus and/or methods of this invention may be employed.